It is well known in the prior art that muscles produce detectable signals. When detected with surface electrodes the myoelectric signal has a useful frequency spectrum initially between 0 Hz and 300 Hz. As the particular muscle becomes fatiqued, the frequency components are compressed to lower frequencies. Accordingly, these frequencies can be monitored, e.g., to determine the effectiveness of muscle therapy, or the level of worker fatigue. E.g., in Fehmi et al. U.S. Pat. No. 3,978,847, computer analysis of a myoelectric signal is carried out to provide real time tracking of its amplitude and frequency spectrum.
Carlo J. DeLuca and W. Berenberg, "A Polar Technique for Displaying EMG Signals", (28th ACEMB, 1975) describes a monitor in which a myoelectric signal is high and low pass filtered and the resulting signals are converted to a pair of rms voltages. The filter cutoff frequencies are adjusted to provide initially equal rms voltages, and the subsequently varying voltages are then measured and displayed on a polar plot. In the actual work on which this publication was based, the initial cutoff frequency adjustment was carried out manually, requiring a recording of the myoelectric signal to be monitored.